A First-principles Study Of Electron Transport Properties Of Light-driven Molecular Switch

Molecular electronics has been highly caused the attention of scientists for their potential applications in the field of nano-size functional material.Using single molecule or molecular cluster,such as single-and multiple-walled carbon nanotubes,single organic molecule,macromolecules and so on,to construct functional electronic devices,and research the electric and optical properties for these molecular devices,this is the main research content of molecular electronics.In recent years,with the development of experimental techniques and theoretical approaches,it is possible to design and measure molecular devices with different functionalities.The single molecule based molecular electronic devices have unique physical properties,including negative differential resistances(NDR),molecular rectification,switches,memories and diodes.Of these devices,the study on the molecular switches has become the world frontier subject because they have the unique features of storing information and transferring information.Molecular switching devices can be divided into two main categories according to the trigger factors:the electrical switches and the optical switches.The former control the conversion between on and off states by an external trigger such as the electric field or the current pulse,and the latter achieve the open and closed state of current by the impulse of light.The optical switch devices have been widely studied for the reason that light is a very attractive external stimulus with its short response time and ease of addressability.In our work,we have investigated the electron transport properties of the molecular motor and the diarylethene using the on-equilibrium Green's function formalism combined with first-principles density functional theory.And discuss their possibility as an optical molecular switch.The outline and the main conclusions of the studies are as follows:].Light-driven rotary molecular motor-based optical molecular switchThe two main forms of the Feringa's second-generation light-driven molecular motor during the 360° rotation,named the anti-folded isomer and the syn-folded isomer.In our calculation,it is found that the current of the syn-folded isomer is always larger than that of the anti-folded isomer,meaning that the conductivity of the molecules alters four times within a rotary cycle.The merit of the periodic photo-induced conductivity tuning makes the molecule a promising candidate for optical molecular switches.The reason is attributed to the change in the effective conjugation length of the two isomers.The significant transmission peak of the syn-folded isomer near the Fermi level comes from the HOMO orbital.The HOMO orbital of the syn-folded isomer is more delocalized than that of the anti-folded isomer.The larger distance from the transmission peak to the Fermi level of the anti-folded isomer leads to its lower conductivity than the syn-folded isomer.2.Diarylethene-based optical molecular switchIn experimental,it is possible to switch diarylethenes with visible light in both directions.In our calculation,it is found that the current through the closed-form is larger than that through the open-form,indicating a switching characteristic.In addition,one NDR response appears at a bias voltage of 0.9V in the closed-form.The lack of any significant peak near the Fermi level accounts for the low conductivity in the open-form.It is attributed to that the HOMO and LUMO orbitals of the open-form are localized.However,the LUMO orbital of the closed-form is delocalized,and the significant transmission peak of the closed-form near the Fermi level comes from the LUMO orbital,which provide good transport channel for electron passing through the molecular junction.The summation of the transmission peak at the bias 0.8V is bigger in the bias window than that of peak at a bias 0.9V.Therefore,a NDR response was observed.